I thought hypertension was caused by a stressful lifestyle. My cat certainly does not
have that.

Hypertension in people is related to several factors, including a lifestyle that
constantly produces stress. Not all of the causes of hypertension in cats have been
identified. However, it does not appear that psychological stress plays a role in
development of this disease as it does in humans.

How do I know if my cat has hypertension?

The most common clinical presentation of feline hypertension is sudden blindness. This
occurs because high pressure in the blood vessels of the retina causes the retina to
detach. Affected cats have widely dilated pupils that do not constrict when exposed to
bright light. These cats run into objects in their path because most of them have no
vision at all.

What causes hypertension in cats

We have identified two major causes of feline hypertension. The first is kidney
failure. The second is heart disease.

How does kidney failure lead to the development of hypertension?

The kidneys are filters that remove waste products from the blood. As a cat gets older,
the kidneys undergo normal aging changes, including the slow accumulation of scar tissue.
This scar tissue causes the kidneys to shrink in size. With every heartbeat, about 20% of
the blood pumped out is delivered to the kidneys. When the kidney shrinks due to the
accumulated scar tissue, it is harder for the blood to filter through. There is a backup
of blood into the arteries and an increase in blood pressure.

One study found that about 60% of cats in old-age kidney failure had hypertension.
Elderly cats not in actual kidney failure may also have hypertension by the same
mechanism.

How does heart failure cause hypertension?

Not all forms of heart failure cause hypertension. However, there are atleast two forms that have been
shown to cause hypertension in cats. Hypertrophic cardiomyopathy is a disease of the heart
muscle with an unknown, cause. It makes the heart muscle thicken, resulting in blood being
pumped more forcefully. This increase in cardiac output of blood results in
hypertension. Hyperthyroidism is another cause of heart disease that may cause
hypertension.

What is hyperthyroidism, and how does it cause hypertension?

The thyroid is a gland located in the neck. It plays a very important role in
regulating the body's rate of metabolism. Hyperthyroidism is a disorder characterized by
the overproduction of thyroid hormone.

When excessive amounts of thyroid hormone are in the circulation, the body's metabolism
speeds up greatly. Many organs are affected by this disease, including the heart. The
heart is stimulated to pump faster and more forcefully, much like hypertrophic
cardiomyopathy. This results in a greater output of blood and high blood pressure. About
80% of cats with hyperthyroidism have high blood pressure, although most of them do not
have blood pressures high enough to cause blindness.

Hyperthyroidism is a fairly common disease of older cats. Although the thyroid gland
enlarges, it is usually a non-malignant change (benign). Less than 2% of hyperthyroid
cases involve a malignant change in the gland.

How is hypertension diagnosed?

Hypertension is suspected in any cat with a sudden onset of blindness, kidney failure,
heart disease (especially hypertrophic cardiomyopathy), or hyperthyroidism. If any of
these are diagnosed, the cat's blood pressure will be checked, if the equipment is
available. If a sudden onset of blindness occurs and the blood pressure is found to be
abnormally high, the other conditions are considered as possible underlying causes. This
means that radiographs (x-rays) of the chest are taken and blood tests are performed for
kidney and thyroid function.

Why don't all veterinarians have blood pressure testing equipment?

Blood pressure is determined with a device that can detect blood flow in arteries.
However, the cat has very small arteries compared to humans. Therefore, the standard blood
pressure equipment used on humans will not work on cats. Only two blood pressure machines
have been found reliable in cats. One costs several hundred dollars and the other several
thousand. Thus, the expense in purchasing this equipment, coupled with the relative
infrequency of hypertension, makes ownership of blood pressure equipment prohibitive for
many veterinarians.

ls there a way to treat it?

There are several drugs that are very effective in treating hypertension in humans.
However, none of these are approved for feline hypertension. Veterinarians have found that
some of these drugs are effective in cats. Although research is still ongoing to determine
the most effective drugs, several have been found that will lower the cat's blood
pressure.

How successful is treatment?

If the cat has blindness due to detached retinas, a medical emergency exists. If the
retinas remain detached for more than a day or two, they probably will not return to their
proper position and become functional again. Therefore, the key to successful treatment is
rapid diagnosis and early administration of the proper medications to lower blood pressure.

If the cat has kidney, heart, or thyroid disease, it is also important to treat those
aggressively. Hyperthyroidism is curable, but hypertrophic cardiomyopathy and old-age
kidney failure are not. However, even those can be managed successfully in many cats.

What is likely to happen in the long term?

If blood pressure can be lowered quickly, some blind cats regain their sight. If blood
pressure can be maintained in the normal range, these cats will retain their sight.
However, the underlying disease that caused hypertension must also be cured or controlled.
Long-term success depends on whether or not this is possible.